TWIST, MV, ACCURACY, STABILITY

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joeb33050 posted this 30 June 2018

 

TWIST, MV, STABILITY AND ACCURACY

 

Many years ago, Loring Hall told me that, sometimes, tipping bullets will form small groups. I have seen this often in the intervening years.

In experimenting with jacketed bullets, attempting to find out why we can’t shoot cast bullets as accurately as jacketed, testing is at muzzle velocities that show the relationship between twist, mv, stability and accuracy.

Starting at very low mv, bullets are wildly inaccurate. As mv increases, bullets then then go through the paper sideways with some accuracy, then go through the paper tipping with some accuracy, and then stop tipping.

Nosler 53 gr. Varmageddon, Titegroup, M12 Savage, Iffland 223 bbl., 14” twist, 5-shot 100 yard groups:

6.5 Titegroup, tipping, 5 shots in 7.5”.

7.0 Titegroup, tipping, 4 shots in 2.7”.

7.5 Titegroup, tipping, 4 shots in 1.15”.

8.0 Titegroup, tipping, 1 group, 1.95”.

8.5 Titegroup, tipping, 6 groups avg. 1.021”.

Hornady .22 68 gr. HPBTM, Titegroup, M12 Savage, Stevens 223 9” twist bbl., 5-shot 100-yard groups.

5.5 Titegroup, 1490 fps, tipping, 1.483” avg./3 groups

6.0 Titegroup, 1628 fps, tipping, .925”/3 groups.

6.5 Titegroup, 1667 fps, tipping, .942/3.

7.0 Titegroup, tipping less, 1932 fps, 1.133/3.

7.5 Titegroup, tipping, 1920 fps, 1.292/3.

8.0 Titegroup, 2006 fps, 1.458/3.

8.5 Titegroup, 2062 fps, 1.383/3.

Hornady .22 68 gr. HPBTM, Titegroup, M10 Savage, 22-250 16.5” bbl., rechambered Stevens 223 bbl., 9” twist bbl., 5-shot 100-yard groups.

5.5 Titegroup, 1248 fps, 1.467” avg./3 groups, 3 sideways.

6.0 Titegroup, 1343 fps, 1.283/3, tipping.

6.5 Titegroup, 1404 fps, .950/3, tipping.

7.0 Titegroup, 1524 fps, .757”/3, tipping.

7.5 Titegroup, 1627 fps, 1.167/3, tipping.

8.0 Titegroup, 1696 fps, 1.042/3, tipping.

8.5 Titegroup, 1764 fps, .800/3, tipping.

Hornady .22 68 gr. HPBTM, Titegroup, M10 Savage, Shilen 9” twist bbl., 5-shot 100-yard groups.

(Photograph attached)

5.5 Titegroup, 1340 fps, most sideways, groups of 1.4” and 1”; 4-shot group of 1.2”

6.0 Titegroup, 1433 fps, some going through sideways, groups of 1.4” and .6”, a 4 shot group of 1.9”.

6.5 Titegroup, 1517 fps, 1 of 15 sideways, others tipping, group-s of 1.0”, .9”, 1.4”

7.0 Titegroup, 1622 fps, tipping, groups of .6”, .9” and .8”.

 7.5 Titegroup, 1713 fps, tipping, groups of 1.05”, 1.2” and .8”

8. 0 Titegoup, 1797 fps, no tipping, 6 groups avg. .838"

8.5 Titegroup, 1872 fps, no tipping, 6 groups avg. .958"

 

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joeb33050 posted this 30 June 2018

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Scearcy posted this 30 June 2018

I have been shooting 243 Win in matches the past two years. Over that period (approx 3,000 rounds) this is what I learned ABOUT MY RIFLE -  YMMV. 

The slower twist rifles tend to be more accurate. This is 1 x 10 for my rifles. 

The most accurate bullets tend to be the heaviest available - 95-105 gr

I have used loads varying from 1450 fps to 1850 fps.

My most accurate load at 100 yards has a mv of 1450 fps and is made of a relatively soft alloy. 40% of the holes in the target show some evidence of tipping. Accuracy was a .83 moa average for an entire year.

This load would not even stay on paper at 200 yards.

1850 fps (with a harder alloy) is better at 200 yards. accuracy from a 10 inch twist brrel is about the same as accuracy from a 9 inch twist barrel at 200 yards

40% of the bullet holes from the 10 inch twist barrel show evidence of tipping at 200 yards.

Bullets from the 9 inch twist barrel show no evidence of tipping at 200 yards

Velocity obviously plays a role in stability although it seems to take a relatively large increase in Vel to eliminate instability with the 10" twist barrel. In my case atmospheric conditions seem to also but that would be another conversation.

Greenhill (which ignores velocity) indicates that 10" twist is not going to stabilize these bullets

Of course jacketed bullets from full power loads are stable but they get almost another 1,000 fps of velocity

Jim

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Ken Campbell Iowa posted this 30 June 2018

good stuff, guys ... dr. mann would be proud of ya ....

we humans like to see patterns ... and even sometimes see patterns that aren't there ( we seek organization ) ...

in joe's groups, i see a pattern that at very low velocity the group is very large.... then as charges go up, groups get smaller ... then as charges go up further, the groups get bigger ..... owch !! .... then smaller again ...whoa !! ....  a more desirable pattern would be that groups just smoothly get a little better with more velocity ...

could this be " barrel vibrations " ?   should we be using * tuners *  ??

***********

and i might mention that i don't think the wobbling bullets know that they are at 100 or 300 yards .... but they might know how long they have been wobbling ...

thanks guys, i am humbled by your efforts  ( yet again ) and ashamed that i myself have nothing of surety concerning cast bullets that i could offer  ...   those truths and i are never in the same place at the same time ... ( g ) ...ken

 

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45 2.1 posted this 30 June 2018

could this be " barrel vibrations " ?  Basic harmonic variation in groups..... quite easy to show this with Unique in a 308.
should we be using * tuners *  ??  I and a couple of others have found some tuners don't really work with lead.............................the Browning Boss being the exception to that.

 

and i might mention that i don't think the wobbling bullets know that they are at 100 or 300 yards .... but they might know how long they have been wobbling ...  Oh, they do......oval bullet holes at 100 yards turn into round holes at 300. It simple shows that velocity should match twist in a good balanced load.

 

 

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JeffinNZ posted this 30 June 2018

45 2.1 might have point there.  I shoot the Lee Cruise Missile 6.5mm bullet in my Carcano and it suffers from "wobbly bottom syndrome" .  Oval holes at all velocities 1400-1800fps BUT good accuracy.  Have yet to shoot beyond 100m however.

Cheers from New Zealand

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Geargnasher posted this 01 July 2018

You got that right, Jeff! That bullet taught me a lot about stability. I pushed it to over 2700 fps, paper patched, in my .270 Winchester and it still yawed at 100 yards, still grouped great though. By 200 it was starting to even out. That was a 10" rot. The Swedish Mauser would just barely make it fly point on at 100 yards, but I either had to quit at 1500 fps or push it over 2100, and the hv groups were never all that great. After cutting the mould down to 150 grains it would stabilize at lower velocity but would still Take over 100 yards to dampen out fully in any of my rifles. I've described that bullet in aggravation more than once as "unstable at any speed".

After all that I pay a lot closer attention to cg/cp relationships when considering a bullet design. The 266469 is easily stabilized.

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Scearcy posted this 01 July 2018

I shoot two 6mm bullets that present an interesting comparison. One is the RCBS 245-95 which is basically a miniature 311299. It weighs 95 gr and is .92" long. I also shoot the NOE 245-105 fn. It actually weighs 95 gr and is .92" long also. This bullet has a long bore ride nose with a flat point.

A 10" twist barrel will stabilize the RCBS bullet out to 200 yards - even with a muzzle velocity as low as 1500 fps. Unfortunately this bullet is not very accurate in my rifles.

As I mentioned in my post above the NOE bullet requires a 9" twist to stabilize but shoots very well (particularly in the 10 " twist barrel) up to the point that I can't keep it on paper.

Some of the best articles I have read on bullet stability refer some bullet forms which are  more stable than others for a given length and weight.

None of the authors offered an easy way to determine if a bullet was inherently unstable because of it "form". Location of the Cg perhaps? There was no consensus as to this most logical attribute.

Jim

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Geargnasher posted this 01 July 2018

A delta cg/cp of 1.8 is about as low as I can get to damp out by 100 yards. 2.0 to 2.3 is much better, usually making round holes before 25 yards as low as 900fps.

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joeb33050 posted this 01 July 2018

A delta cg/cp of 1.8 is about as low as I can get to damp out by 100 yards. 2.0 to 2.3 is much better, usually making round holes before 25 yards as low as 900fps.

What is "A delta cg/cp of 1.8 ", how is it measured/calculated, and what does it mean? Why should we care if 25 yard holes are round at 900 fps?  Stability is a function of twist, shape, velocity (speed, actually), bullet and air density. Was Gebst?

joe b.

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Geargnasher posted this 02 July 2018

Jim, most of the more popular bullet design programs indicate the locations of the two points. Empirical tests of several known bullets show the trend of stability I indicated above, with the cg always needing to be closer to the base by a certain percentage than the cp for any spin-stabilized projectile. Since both factors can and do change slightly after the bullet makes its way through the barrel, actual shooting is the only way to get positive data. Look at the drawing for your noe bullet, you will note that the difference (delta) between cg and cp is 1.73%. Sufficient at decent speed but not great for bucking the transonic region.

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Geargnasher posted this 02 July 2018

I can't seem to edit from a mobile device. Add to the above that I haven't worked out a positive, calculable, relationship between speed, rate of twist, and delta cg/cp, but one is there. My 1.8 minimum is for normal-for-caliber ROTs with supersonic bullets.

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joeb33050 posted this 02 July 2018

cg is easy, cp ain't. I can't find a simple way to calculate cp, calculation is done by using a series of shapes and tables of values for these shapes. Why do we care about cg and cp? Stability of bullets/twist " is easily calculated using Greenhill. Criticism of Greenhill seems connected to the critic's failure to read the available material.

I have never seen or read a reliable report of the failure of a bullet to stabilize with twist = Greenhill min., other conditions met.

(cg-cp)/cg, and beware negative solutions-which are hard for me to imagine for an object that goes through a gun barrel.

joe b.  

 

 

 

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Larry Gibson posted this 02 July 2018

Concur with joe.  I think there is a tendency to over think a problem.  Using Greenhill and/or Miller stability formula's I have never had a stability problem.  If a bullet is giving undesired stability problems use another (usually shorter bullet), push it faster or use in a faster twist if possible. Some bullets such as the 6.5 CM are just too much of what some consider to be a good thing......and just don't work out in reality. I prefer not to continue dealing with unstable bullets.

LMG

Concealment is not cover.........

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45 2.1 posted this 02 July 2018

Reality.......seriously, the 6.5 CM is a little shorter than the original FMJ as loaded in the Swedish Mauser. It's the way people load it that's the problem (along with the fact that Lee seldom cuts it to the original specifications). The original single cavity GB molds produce equal accuracy as compared to the old RN Swedish military loading's. It seems unstabile because people want to load it slow with a pinch of powder. Most of those loads like that will produce oblong holes at 100 yards....... take them out to 300 yards and they're round holes. A fellow that used to post on CB said the Swede is a graduate level rifle...... he was right.

As far as the cg/cp discussion, very few of you shoot far enough to worry about it.

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pat i. posted this 03 July 2018

It seems some things never change. Now I know why I've pretty much given up on cast bullet forums.

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joeb33050 posted this 03 July 2018

See first post for more info.

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Larry Gibson posted this 03 July 2018

Yup, plenty of good information via actual results in joe's first post.  Get's to the point of what he's saying.

LMG

Concealment is not cover.........

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Ed Harris posted this 03 July 2018

Joe, your best post ever!

Agrees with my experience years ago when we were down-loading 5.56mm military bullets for terminal ballistic testing.

Rotational velocity decays more slowly than translational velocity, so if you want to simulate a 300 meter impact with 5.56mm M193 it works out well to use the 7" twist M4 barrel with about 7-8 grains of Unique for about 2000 fps and for 500 metres about 5-5.5 grains for 1300 fps or so. 

Wastes much less ammo than trying to hit the gelatin block at real range.

FYI the minimum charge which exited the M4 barrel all the time was 4 grains of Unique or 3 grains of Bullseye for about 1000 fps.

Useful to know if anybody has a .223 with a "can." 

 

73 de KE4SKY In Home Mix We Trust From the Home of Ed's Red in "Almost Heaven" West Virginia

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pat i. posted this 03 July 2018

See first post for more info.

It wasn't your posts I was referring to Joe. I like posts that have actual results.

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joeb33050 posted this 03 July 2018

See first post for more info.

It wasn't your posts I was referring to Joe. I like posts that have actual results.
All is well, Pat. Your 223 bbl. shoots shorter/slower bullets great.
joe b.

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